In a conventional steering control system for a vehicle especially for an automobile, an operation angle of a steering wheel (i.e. a steering wheel operation angle) has been publicly known to be transmitted to a vehicle wheel to be steered without being varied. That is, the steering wheel operation angle is always transferred at one for one rate (1:1) for a vehicle wheel steering angle. However, recent developments have lead to the vehicle steering control system mounting a variable steering angle conversion ratio mechanism by which a conversion ratio for converting the steering wheel operation angle to the vehicle wheel steering angle (hereinafter, referred to as a steering angle conversion ratio) is varied in accordance with vehicle driving conditions such as a vehicle speed. At a time of vehicle high-speed travel, it is preferable to set the steering angle conversion ratio at a relatively small ratio. In this case, the steering of vehicle wheel angle can be prevented from being rapidly increased in response to increase of the steering wheel operation angle, thereby enabling to stabilize the vehicle high-speed travel. On the other hand, at a time of vehicle low-speed travel, it is preferable to set the steering angle conversion ratio at a relatively large ratio. In this case, the vehicle wheel can be steered to the maximum steering extent possible with the small steering wheel operation angle. That is, the driver does not have to operate the steering wheel many times so as to steer the vehicle wheel to the maximum steering extent possible. Therefore, it makes easier to perform driving performance, which requires the vehicle wheel to be steered at a relatively large steering angle, such as parking to a garage, parallel parking, and pulling over to the kerb.
One type of variable steering angle conversion ratio mechanism has been disclosed in a Japanese Patent Laid-Open Publication published as No. 1999-334604. Disclosed above has a geared transmitting unit which directly connects a shaft connected to the steering wheel (i.e. a steering wheel shaft) and a vehicle wheel steering shaft with a variable gear ratio. However, the construction of the geared transmitting unit may become complicated.
In another Japanese Patent Laid-Open Publication published as No. 1999-334628, disclosed is a vehicle steering control system with a variable steering angle conversion ratio mechanism in which the vehicle wheel steering shaft is driven for its rotation by an actuator such as a motor. More particularly, a target vehicle wheel steering angle is computed based upon the steering wheel operation angle detected by an angle detecting unit and the steering angle conversion ratio determined in accordance with the vehicle driving conditions. The actuator such as the motor rotates the vehicle wheel steering shaft, which is mechanically disconnected from the steering wheel shaft, so as to steer the vehicle wheel at the target vehicle wheel steering angle.
However, when the steering wheel shaft and the vehicle wheel steering shaft are mechanically disconnected as described above, following problem may arise.
Namely, when the vehicle driving operation is terminated leaving the steering wheel steered at a certain steering angle, the actuator also terminates the rotation of the vehicle wheel steering shaft at that time. At this point, the vehicle wheel steering shaft remains at an angle position at the time of the termination of the vehicle driving operation (i.e. a final angle position). The final angle position is stored in a control unit of the actuator. When the vehicle secondly starts, the vehicle wheel steering shaft is driven again for its rotation by the actuator from the stored final angle position which has been assumed as an initial angle position for this second vehicle operation. However, there may be a case as one of likely problems, that the final angle position can not be stored in the control unit of the actuator for some reasons. In this case, the vehicle wheel steering shaft is disabled to be normally driven for its second rotation by the actuator.
A need thus exists for providing an improved vehicle steering control system capable of accurately identifying whether or not the final angle position at the time of the termination of the vehicle driving operation was successfully written in the control unit.